1. Field of the Invention
This invention relates to a method and apparatus for the fabricating of high-density monolithic refractory metal and alloy billets, and more particularly to a two step process referred to as Combustion Synthesis Assisted Hot Explosive Compaction, and a vessel for containing and withstanding said reaction.
2. Discussion of Related Art
The development of improved refractory alloys comprised of tungsten, molybdenum, and tantalum is an ongoing research project. In general, the development of these alloys requires a continuous matrix phase of one of the components, good intergrain bonding between the primary and matrix phases, and good machinability. The metallurgy of the improved composition of the alloy requires that no intermetallics are formed.
Tungsten heavy alloys are most effectively produced by liquid phase sintering. Most of the existing liquid phase sintering technology for fabricating tungsten heavy alloys has been focussed on developing two phase composites, consisting of spheroidal tungsten particles embedded in a nickel--iron, or a copper--nickel matrix. Recently, there has been a renewed interest for extending the liquid phase sintering method to incorporate other matrix materials, such as titanium,(Ti-6Al-4V), zirconium, hafnium, and other steel alloys. However, it has been determined, when using zirconium, hafnium or iron with this method, undesirable intermetallic phases may form if a critical temperature is exceeded.
Powder metallurgy methods are often considered when conventional forming routes, such as melting, casting and forging, are not appropriate. Such methods are more economical due to the fact that there is a reduced need for machining. Powder metallurgy methods produce products with near net-shape and practically finished dimensions. Recent developments in the preparation of powder precursors allow for greater control over the grain size and improved component distribution, resulting in a more homogeneous structure.
For the fabrication of a new class of tungsten heavy alloys, a method similar to liquid phase sintering is desired. In addition to conventional powder metallurgy fabrication techniques, such as liquid phase sintering, other processes have been considered. Examples of such processes include solid state sintering and coextrusion through a die, and the use of explosive compaction to consolidate and sinter the powdered precursors into full density products.
In consideration of alternative methods of fabricating tungsten heavy alloys, the explosive compaction method has the capability of producing not only consolidation of metal and ceramic powder, but equilibrium and non-equilibrium structures as well. Such tungsten heavy alloys are produced through exposure to and passage of a shock wave. The shock wave is generated by an impact of a projectile or a flyer plate, produced by compressed gas or detonation of an explosive. In producing the conventional compaction geometries, the application of the shock wave can be in either a planar or cylindrical configuration. As the shock wave propagates through the metal or ceramic powder, it densities the powder and produces bonding between adjacent articles.
The rapid densification rate produced by the shock wave, can result in low nonuniform densities, poor interparticle bonding and severe cracking of the structures. Furthermore, a rise in temperature of the powders occurs as a result of irreversible work occurring during the consolidation of distended solids. This rise in temperature has frequently been found to be insufficient in the development of bonding of the particles.
There has been an attempt to address the problem associated with the sudden rise in temperature associated with the shock wave in a report "Hot Explosive Pressing of Powders," by Gorobtsov and Roman. This report suggests a method known as Hot Explosive Compaction (HEC) which includes preheating the powder prior to exposure to the shock wave. The advantage of preheating the powder is to decrease its yield strength. This results in increasing ductility and allows for greater thermal softening.
A variation of the HEC method for consolidating tungsten-based alloys is the Combustion Synthesis Assisted HEC (CSA-HEC). This technique requires a self-propagating high-temperature synthesis reaction (SHS), thereby producing a chemical furnace, for preheating the powder sample prior to compaction. The SHS involves the synthesis of ceramic and intermetallic materials directly from their elemental precursors, by means of propagation of a solid--solid combustion front, through a green powder compact. Following initiation of the SHS process, the heat of the reaction is sufficient to sustain the reaction until all of the reactants have been consumed. Such reactions are characterized by rapid reaction rates, temperatures of approximately 3000 degrees C., and at times violent evolution of impurities trapped on the reactants.
An advantage of a combustion synthesis process, such as SHS, is that it requires a relatively small energy input, and the processing time is reduced to seconds, as opposed to hours. In addition, temperatures much higher than those of conventional furnaces can be achieved by such a process. Furthermore, the CSA-HEC method eliminates the remotely controlled transfer system used in the conventional HEC process for moving the heated powders from the furnace to the explosive assembly. Placing the heating source around the sample allows for heating and consolidation of the powder in the same fixture.
There are significant differences between liquid phase sintering techniques, HEC and CSA-HEC processes. Liquid phase sintering allows for the tungsten to dissolve either partially or completely into the matrix, thereby making the initial metal powder morphology to be of less importance. In the past, application of the HEC process was limited to homogeneous systems, such as pure and prealloyed metals and ceramic powders. The long preheating cycle is likely to cause the dissolution of the tungsten into the matrix to form an extensive solid solution. However, in the CSA-HEC process, the length of the heating cycle is reduced. Therefore, it is likely that the solid solution formation would be limited to interfacial regions leaving the original tungsten grain morphology intact. The heating cycle characteristic is critical if the desirable properties are to be retained, such as size distribution and morphology of the refractory precursor. It should be noted that the rapid temperature quench may reduce impurity segregation at interfaces or grain boundaries. Adjustment of available heat and peak temperatures during the preheating cycles can be made to limit sintering, dissolution and melting in each phase in order to maintain the precursor structure and morphology. Thus, melting of the matrix metal can occur while preventing complete dissolution of the primary component.
In "Hot Explosive Pressing of Powders," by Gorobstov and Roman, the basic issues associated with HEC are defined. The powders are preheated in a furnace, while an explosive, a plane wave generator and a base plate are mounted and assembled. Following arming of the detonator, the furnace is opened and a container holding the hot-powder is pushed along a guidance system until it is placed beneath the explosive charge. The time lapse between transfer of the container from the furnace until detonation of the explosive was less than 20 seconds. This technique consolidated large 10 mm .times.100 mm .times.180 mm flat plates. Tensile and Charpy impact strength tests of the HEC samples resulted in high ductility and toughness. This implies that the HEC compacts were strong enough for machining and mechanical working, without a need for sintering. The authors concluded that the HEC above recrystalization temperature of the material resulted in full density compacts. In addition, they speculated that in the course of extensive plastic deformation, the underlying mechanism responsible for this structure was dynamic recrystalization caused by high pressures and high temperatures.
The HEC process has also been addressed in "Hot Explosive Compaction of Metal Powders," by Bhalla. However, in place of the planar compaction used by Gorobtsov and Roman, Bhalla uses a cylindrical compaction configuration. The precursor powder mixture is placed in a sealed tube. The tube is then preheated in a furnace, quickly removed, transferred and dropped into the compaction apparatus, and consolidated to full-density. As a result, above recrystalization temperature HEC was controlled by the impact of the energy input, and that the temperature had a small influence on the density of the sample. Due to high temperature malleability and plasticity of powder, it was determined that the energy requirements were found to be substantially lower than when explosive compaction was performed at room temperature. Again, it was concluded that the compacts produced by the HEC method have superior mechanical properties and did not require additional sintering.
U.S. Pat. No. 4,655,830 to Akashi et al. and U.S. Pat. No. 5,139,720 to Takeda also disclose the application of a chemical furnace for enhancing the properties of SHS synthesized ceramics. The Akashi et al. patent further discloses an explosive compaction fixture designed to hold multiple sample capsules. The fixture is fitted with an external heating unit. Shock induced damage is reduced by positioning capsules off high-symmetry axes and attaching a momentum trap to the base of the fixture. After the heating element elevates the temperature of the entire steel fixture, the explosive container is allowed to slide onto the steel fixture, and is then remotely detonated. Among the unique features of this design, is the ability to synthesize twelve samples simultaneously. However, compaction of all twelve samples to full density is not likely to be achieved due to mechanical constraints imposed by the plurality of capsules. Results of the procedure were compared to compactions of different particle size, initial powders, and admixtures with graphite, silicon and Ti+C at temperatures of 600.degree. C. and 700.degree. C. The samples compacted at higher temperatures had less cracking, improved bonding, and increased hardness. It was found that cracking was associated with increase in grain size.
Finally, U.S. Pat. No. 5,114,645 to Niiler et al. discloses a two step process for producing fully dense ceramics. This method involves synthesis of the samples followed by compaction after their formation. In relation to the CSA-HEC method, this method uses preheating followed by compaction.
The following disclosure relates to the two-step process of Niiler et al. in combination with the chemical furnace disclosure of Akashi et al. and Takeda. This combination results in a unique two-step explosive compaction process. The SHS synthesized material is used only as a heat source and functions to provide the heat necessary to increase the temperature of an enclosed metal or alloy sample.